cannot use the `?` operator in a function that returns `()`

[zzke]

Im trying to compile the example given in the README and it raise some errors as follows

system: macOS Big Sur rust version: 1.55.0

error[E0277]: the ? operator can only be used in a function that returns Result or Option (or another type that implements FromResidual) --> src/main.rs:13:35	11	/ fn main() { 12		let url = "mysql://root:password@localhost:3307/db_name"; 13		let opts = Opts::from_url(url)?;		^ cannot use the ? operator in a function that returns () 14		let pool = Pool::new(opts)?; ...	58		println!("Yay!"); 59		}		_- this function should return Result or Option to accept ?

= help: the trait `FromResidual<Result<Infallible, mysql::UrlError>>` is not implemented for `()`

note: required by from_residual

error[E0277]: the ? operator can only be used in a function that returns Result or Option (or another type that implements FromResidual) --> src/main.rs:14:31	11	/ fn main() { 12		let url = "mysql://root:password@localhost:3307/db_name"; 13		let opts = Opts::from_url(url)?; 14		let pool = Pool::new(opts)?;		^ cannot use the ? operator in a function that returns () ...	58		println!("Yay!"); 59		}		_- this function should return Result or Option to accept ?

= help: the trait `FromResidual<Result<Infallible, mysql::Error>>` is not implemented for `()`

note: required by from_residual

error[E0277]: the ? operator can only be used in a function that returns Result or Option (or another type that implements FromResidual) --> src/main.rs:16:35	11	/ fn main() { 12		let url = "mysql://root:password@localhost:3307/db_name"; 13		let opts = Opts::from_url(url)?; 14		let pool = Pool::new(opts)?; 15		16		let mut conn = pool.get_conn()?;		^ cannot use the ? operator in a function that returns () ...	58		println!("Yay!"); 59		}		_- this function should return Result or Option to accept ?

= help: the trait `FromResidual<Result<Infallible, mysql::Error>>` is not implemented for `()`

note: required by from_residual

error[E0277]: the ? operator can only be used in a function that returns Result or Option (or another type that implements FromResidual) --> src/main.rs:24:8	11	/ fn main() { 12		let url = "mysql://root:password@localhost:3307/db_name"; 13		let opts = Opts::from_url(url)?; 14		let pool = Pool::new(opts)?; ...	24		)")?;		^ cannot use the ? operator in a function that returns () ...	58		println!("Yay!"); 59		}		_- this function should return Result or Option to accept ?

= help: the trait `FromResidual<Result<Infallible, mysql::Error>>` is not implemented for `()`

note: required by from_residual

error[E0277]: the ? operator can only be used in a function that returns Result or Option (or another type that implements FromResidual) --> src/main.rs:43:6	11	/ fn main() { 12		let url = "mysql://root:password@localhost:3307/db_name"; 13		let opts = Opts::from_url(url)?; 14		let pool = Pool::new(opts)?; ...	43		)?;		^ cannot use the ? operator in a function that returns () ...	58		println!("Yay!"); 59		}		_- this function should return Result or Option to accept ?

= help: the trait `FromResidual<Result<Infallible, mysql::Error>>` is not implemented for `()`

note: required by from_residual

error[E0277]: the ? operator can only be used in a function that returns Result or Option (or another type that implements FromResidual) --> src/main.rs:52:10	11	/ fn main() { 12		let url = "mysql://root:password@localhost:3307/db_name"; 13		let opts = Opts::from_url(url)?; 14		let pool = Pool::new(opts)?; ...	52		)?;		^ cannot use the ? operator in a function that returns () ...	58		println!("Yay!"); 59		}		_- this function should return Result or Option to accept ?

= help: the trait `FromResidual<Result<Infallible, mysql::Error>>` is not implemented for `()`

note: required by from_residual

For more information about this error, try rustc --explain E0277. error: could not compile rust-mysql-simple due to 6 previous errors

[blackbeam]

Hi.

README.md assumes that a reader is somewhat familiar with basic aspects of Rust. Please read the Error Handling section of the Rust Book (especially A Shortcut for Propagating Errors: the ? Operator) as well as the Error Handling section of the Rust by Example (especially Introducing ? and Using Result in main).

You can update your main definition to fix the error (similar definition is actually used in the example):

fn main() -> Result<(), Box<dyn std::error::Error>> {
    // ...
}

Anyway, thank for report. I'll update the README example with the fake definition of main to avoid this confusion in future.

[zzke]

thanks!